Because of their unique combination of mechanical properties, thermoplastic polyurethanes (hereinafter TPU) have been extensively used in a variety of molding applications. Articles molded therefrom have met the stringent engineering criteria governing applications in the automotive, electrical, mechanical equipment and footwear industries. However, the flammability rating of TPU (UL-94-1/8", V-2) limits their utility in certain applications where a greater resistance to flame is required. Halogenated fire retardants, singly or in combination with antimony oxide have been reported to improve the flame retardance of these resins but are known to be unsuitable for some applications where corrosion upon evolution of hydrogen halides attended upon combustion is a determining factor.
Thermoplastic polyphosphonates are characteristically of exceptionally high rating of flame resistance- see for instance German Patent Application No. P 29 44 093.5. Due in part to their level of mechanical properties however, thermoplastic polyphosphonates are beneficially used in a narrower field of applications.
Polyblends of TPU and certain thermoplastic polyphosphonates purposed at combining their individually useful properties have been reported (U.S. Ser. No. 160,646 filed June 18, 1980 now U.S. Pat. No. 4,331,614). It has been the experience of those skilled in the art that certain other thermoplastic polyphosphonates, desirable as components in a blend with TPU are not compatible and do not easily blend therewith. Attempts to prepare such blends invariably led to a poorly dispersed inhomogeneous mixture unsuited for molding applications.